Magnetic Compton Profiles Research Articles

Electronic properties and quantitative mapping of magnetic moments of rare-earth hard magnet SmCo5: First principles computations and inelastic scattering

Looking at the limitations of neutron spectroscopy in exploring properties of rare earth hard magnet SmCo5, inelastic scattering for charge and magnetic Compton profiles are attempted. The experimental momentum densities using 661.65 keV γ-ray energy are compared with the theoretical Compton profiles (CPs) deduced using linear combination of atomic orbitals (LCAO) with different exchange and correlation potentials and also spin polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) calculations. The anisotropies in the computed CPs, along the principal directions, show high momentum densities along the c-axis [00.1] of SmCo5 than [10.0] and [11.0] directions. An overall better agreement of CP computed using LCAO-GGA and that from experiment is observed which shows more appropriateness of GGA-PBE scheme for the exchange and correlation energies for SmCo5. The small value of charge transfer from Sm → Co and positive value of overlap Mulliken population dictate dominance of covalent bonding in SmCo5. Moreover, available experimental data on c-axis oriented magnetic CP (MCP) of SmCo5 have been compared with the present SPR-KKR calculations. It is seen that present SPR-KKR based MCP is in resemblance with the available experimental MCP. A decomposition of experimental MCP in to its constituent profiles depict that the spin momentum of Co and Sm are aligned anti-parallel while the diffuse contribution aligns opposite to the absolute spin moment. Comparison of amplitude and direction of site specific spin moments in SmCo5 is made with the present SPR-KKR and available experimental and theoretical data.

Inelastic Compton scattering: Tool to validate quantum mechanical potential energies

Inelastic γ-ray scattering (Compton scattering) is a distinguished tool to explore the electronic and magnetic response of various substances and is also an emphatic technique to validate the quantum mechanical exchange and correlation potentials used in the DFT calculations. Charge and magnetic Compton profiles are used to deduce the electron/spin momentum densities in functional materials like dichalcogenides, ferrites, ferroelectrics, chromates, Heusler alloys, oxides, etc. In this review, besides salient features of basic principles of Compton scattering, we have included representative studies on Compton profiles and relative nature of bonding in lead titanate/lead zirconate. Temperature dependency of spin moments using magnetic Compton profiles of Pr doped NiFe2O4 system has also be discussed. Recent work on charge and magnetic Compton scattering of variety of materials is reviewed.

Influence of Pr doping in non-stoichiometric NiFe2O4 on magnetic response and electronic properties: Magnetic Compton scattering and ab-initio studies

Magnetic Compton profiles of 5 % and 15 % Pr-doped non-stoichiometric NiFe2O4 have been measured using synchrotron radiations of energy 182.65 keV at SPring-8, Japan. An analysis of MCPs measured at 6 and 300 K show that the total spin moment decreases with increasing temperature and Pr concentration in Ni0.9-xPrxFe2.1O4 which is consistent with the magnetic moment (spin + orbital) as observed from magnetization measurements data. Spin moments at different sites are deduced using decomposition of MCPs in the elemental profiles of Ni, Pr-4f, Fe and diffused electrons. Spin moments at Ni-site have been found to play crucial role in emergence of absolute spin moments. Spin moments at Pr-site is aligned antiparallel to that of Ni and Fe sites. The orbital magnetic moment is also deduced from the subtraction of MCP based spin moments from the magnetization data. Theoretical FP-LAPW based total and atom specific spin moments are found in consonance with the MCP based data. Total and spin projected density of states are also computed using the FP-LAPW method to validate role of Pr-4f and Fe-3d (Oh) states in amending the total magnetic moment with Pr concentration. The M−H based saturation magnetization derived at 6 and 300 K temperature show that the saturation magnetic moments reduce from 2.21 → 2.05 μB/f.u. at 6 K and 2.10 → 1.98 μB/f.u. at 300 K with increase in Pr concentration from 5 % → 15 %. On the basis of hysteresis curve and coercive force, both the doped compounds show soft magnetism.

Understanding the effect of Ni and Co doping in magnetic properties of La0.7Ca0.3MnO3: Magnetic Compton spectroscopy and DFT strategies

Magnetic Compton profiles (MCPs) of La0.7Ca0.3Mn0.95(Ni/Co)0.05O3 between 8 and 250 K have been measured using high energy inelastic scattering at SPring-8, Japan. For a quantitative analysis on amendment of magnetic properties, temperature-dependent experimental MCPs are decomposed into constituent profiles related to Mn, Ni/Co and itinerant (diffuse) electrons. Decomposed profiles unambiguously show that Mn-3d electrons are ferromagnetically coupled with Ni/Co ions while itinerant electrons are linked anti-ferromagnetically with Mn-3d and Ni/Co-3d electrons. An annotation of MCP and VSM data depicts that the orbital magnetic moments are anti-ferromagnetically coupled to total spin moments. Variation in magnetic response in undoped and Ni/Co doped perovskites is devoted to formation of mixed valence states of Mn atom. The spin contribution at different sites are found to be in tune with present density functional theory (DFT) based calculations which are thoroughly analyzed on the basis of changes in the eg and t2g states of Mn, Ni/Co caused by Jahn-Teller distortion. Present MCP data and DFT calculations set guidelines to probe the reliable origin to examine magnetic response and half-metallic properties of such manganites.

Symmetry of Wavefunction at the Interface of Fe/MgO Magnetic Tunneling Junction

We measured the anisotropy of Magnetic Compton profiles (MCP) in the Fe/MgO multilayers and compared it with band structure calculations. At the Fe/MgO interface, the |m| = 1 state in the minority band is suppressed, which in turn promotes the spin-polarized occupation of the |m| = 1 state. At the Fe/FeO interface of intentionally oxidized Fe/MgO multilayers, the occupations of the magnetic quantum number are almost equal (spherical) in the majority band and minority band, and therefore, the spin-polarization occupancy is also almost equal (spherical). These results contribute to the material design for MTJs as high-performance spintronic devices.

Hidden k-Space Magnetoelectric Multipoles in Nonmagnetic Ferroelectrics.

In condensed matter systems, the electronic degrees of freedom are often entangled to form complex composites, known as hidden orders, which give rise to unusual properties, while escaping detection in conventional experiments. Here we demonstrate the existence of hidden k-space magnetoelectric multipoles in nonmagnetic systems with broken space-inversion symmetry. These k-space magnetoelectric multipoles are reciprocal to the real-space charge dipoles associated with the broken inversion symmetry. Using the prototypical ferroelectric PbTiO_{3} as an example, we show that their origin is a spin asymmetry in momentum space resulting from the broken space inversion symmetry associated with the ferroelectric polarization. In PbTiO_{3}, the k-space spin asymmetry corresponds to a pure k-space magnetoelectric toroidal moment, which can be detected using magnetic Compton scattering, an established tool for probing magnetism in ferromagnets or ferrimagnets with a net spin polarization, which has not been exploited to date for nonmagnetic systems. In particular, the k-space magnetoelectric toroidal moment combined with the spin-orbit interaction manifest in an antisymmetric magnetic Compton profile that can be reversed using an electric field. Our work suggests an experimental route to directly measuring and tuning hidden k-space magnetoelectric multipoles via specially designed magnetic Compton scattering measurements.

Magnetic Compton Scattering Study of Li-Rich Battery Materials

The redox process in a lithium-ion battery occurs when a conduction electron from the lithium anode is transferred to the redox orbital of the cathode. Understanding the nature of orbitals involved in anionic as well as cationic redox reactions is important for improving the capacity and energy density of Li-ion batteries. In this connection, we have obtained magnetic Compton profiles (MCPs) from the Li-rich cation-disordered rock-salt compound LixTi0.4Mn0.4O2 (LTMO). The MCPs, which involved the scattering of circularly polarized hard X-rays, are given by the momentum density of all the unpaired spins in the material. The net magnetic moment in the ground state can be extracted from the area under the MCP, along with a SQUID measurement. Our analysis gives insight into the role of Mn 3d magnetic electrons and O 2p holes in the magnetic redox properties of LTMO.

Magnetic Compton profiles of Ni beyond the one-particle picture: Numerically exact and perturbative solvers of dynamical mean-field theory

We calculated the magnetic Compton profiles (MCPs) of Ni using density functional theory supplemented by electronic correlations treated within dynamical mean-field theory (DMFT). We present comparisons between the theoretical and experimental MCPs. The theoretical MCPs were calculated using the KKR method with the perturbative spin-polarized T-matrix fluctuation exchange approximation DMFT solver, as well as with the full potential linear augmented planewave method with the numerically exact continuous-time quantum Monte Carlo DMFT solver. We show that the total magnetic moment decreases with the intra-atomic Coulomb repulsion $U$, which is also reflected in the corresponding MCPs. The total magnetic moment obtained in experimental measurements can be reproduced by intermediate values of $U$. The spectral function reveals that the minority X$_2$ Fermi surface pocket shrinks and gets shallower with respect to the density functional theory calculations.

Evaluation of Magnetic Moment of Spins in Rare Earth Nitrides by Magnetic Compton Profile Measurements

Evaluation of Magnetic Moment of Spins in Rare Earth Nitrides by Magnetic Compton Profile Measurements

Temperature dependence of the microscopic magnetization process of Tb12Co88 using magnetic Compton scattering

Abstract Tb-Co perpendicular magnetization films are mother materials for perpendicular magnetic recording media. Since the magnetic recording technique is based on magnetic switching of a material, investigating a magnetic the response of the material to a magnetic field is important. The magnetic response of the entire system is the net of each microscopic response of the spin magnetic moment and orbital magnetic moment of the elements. Herein we study the magnetization processes of the spin, orbital, and the element-specific magnetic moments by measuring the magnetic field dependence of the magnetic Compton profile of a Tb12Co88 film. The spin and orbital magnetic moments have opposite orientations. Similarly, Tb and Co magnetic moments have opposite directions. When the total magnetic moment reverses, the orientation of spin and orbital magnetization as well as Tb and Co reverse. Decreasing the temperature, the fluctuation of the magnetic moment of Tb in the perpendicular direction decreases. So the cancelation between the Tb and Co moments reduces total magnetic moment.

Magnetic response of Nd-doped nickel ferrites using magnetic Compton scattering and XPS measurements

Spin-polarized electron momentum densities of Nd-doped nickel ferrites (Ni1−xNdxFe2O4 where , 0.24) at 8 and 300 K temperatures are presented. The measurements have been made using the magnetic Compton scattering set-up available at SPring-8, Japan. The site-specific spin moments which contribute to the formation of the total spin moment are deduced by decomposition of the magnetic Compton profile (MCP) into constituent profiles. An increase in the spin moment at Fe site and a reduction at Ni site with Nd doping, as revealed by MCP data, are explained on the basis of the cationic redistribution at octahedral and tetrahedral sites using x-ray photoelectron spectroscopy (XPS) analysis. The XPS measurements suggest the emergence of Fe2+ ions in Nd-doped samples to maintain the charge neutrality. It is found that the net spin moment at the Nd site is antiparallel to that at Ni and Fe sites. The orbital magnetic moment (deduced from the difference of magnetic Compton and total magnetization data) is also discussed.

Magnetic Compton scattering study of grain-oriented silicon steels

Magnetic Compton profiles (MCPs) of grain-oriented silicon steels with different Si content were measured by a magnetic Compton scattering (MCS) technique. The profile features of the MCP difference between high permeability grain-oriented and conventional grain-oriented steels were similar to those of the theoretical sixth band related to the 3d conduction electrons of Fe. A comparison of the calculated eddy current losses between two samples revealed that Si content affects the contribution degree of 3d conduction electrons to eddy current losses. This brief note presents the relationship between electronic structure and eddy current loss.

Joint refinement model for the spin resolved one-electron reduced density matrix of YTiO3 using magnetic structure factors and magnetic Compton profiles data.

In this paper, we propose a simple cluster model with limited basis sets to reproduce the unpaired electron distributions in a YTiO3 ferromagnetic crystal. The spin-resolved one-electron-reduced density matrix is reconstructed simultaneously from theoretical magnetic structure factors and directional magnetic Compton profiles using our joint refinement algorithm. This algorithm is guided by the rescaling of basis functions and the adjustment of the spin population matrix. The resulting spin electron density in both position and momentum spaces from the joint refinement model is in agreement with theoretical and experimental results. Benefits brought from magnetic Compton profiles to the entire spin density matrix are illustrated. We studied the magnetic properties of the YTiO3 crystal along the Ti-O1-Ti bonding. We found that the basis functions are mostly rescaled by means of magnetic Compton profiles, while the molecular occupation numbers are mainly modified by the magnetic structure factors.

Magnetic Compton profiles of disordered Fe0.5Ni0.5 and ordered FeNi alloys

We study the magnetic Compton profile (MCP) of the disordered Fe$_{0.5}$Ni$_{0.5}$ and of the ordered FeNi alloys and discuss the interplay between structural disorder and electronic correlations. The Coherent Potential Approximation is employed to model the substitutional disorder within the single-site approximation, while local electronic correlations are captured with the Dynamical Mean Field Theory. Comparison with the experimental data reveals the limitation of local spin-density approximation in low momentum region, where we show that including local but dynamic correlations the experimental spectra is excellently described. We further show that using local spin-density approximation no significant difference is seen between the MCP spectra of the disordered Fe$_{0.5}$Ni$_{0.5}$ and a hypothetical, ordered FeNi alloy with a simple cubic unit cell. Only by including the electronic correlations, the spectra significantly separate, from the second Brillouin zone boundary down to zero momenta. The difference between the MCP spectra of ordered and disordered alloys is discussed also in terms of the atomic-type decompositions. Finally based on the presented calculations we predict the shape of the MCP profile for the ordered FeNi alloy along the [111] direction.

Development of a joint refinement model for the spin-resolved one-electron reduced density matrix using different data sets.

The paper describes a joint refinement model of the spin-resolved one-electron reduced density matrix using simultaneously magnetic structure factors and magnetic directional Compton profiles. The model is guided by two strategies: (i) variation of basis functions and (ii) variation of the spin population matrix. The implementation for a finite system is based on an expansion of the natural orbitals on basis sets. To show the potential benefits brought by the joint refinement model, the paper also presents the refinement results using magnetic structure factors only. The joint refinement model provides very satisfactory results reproducing the pseudo-data. In particular, magnetic Compton profiles have a strong effect not only on the off-diagonal elements of the spin-resolved one-electron reduced density matrix but also on its diagonal elements.

Magnetic Compton scattering study of Laves phase ZrFe2 and Sc doped ZrFe2: Experiment and Green function based relativistic calculations

Spin momentum densities of ferromagnetic ZrFe2 and Zr0.8Sc0.2Fe2 have been measured using magnetic Compton scattering with 182.65 keV circularly polarized synchrotron radiations. Site specific spin moments, which are responsible for the formation of total spin moment, have been deduced from Compton line shapes. At room temperature, the computed spin moment of ZrFe2 is found to be slightly higher than that of Sc doped ZrFe2 which is in consensus with the magnetization data. To compare the experimental data, we have also computed magnetic Compton profiles (MCPs), total and partial spin projected density of states (DOS) and the site specific spin moments using spin-polarized relativistic Korringa-Kohn-Rostoker method. It is observed that the spin moment at Fe site is aligned antiparallel to that of Zr site in both ZrFe2 and Zr0.8Sc0.2Fe2. The MCP results when compared with vibrating sample magnetometer based magnetization data, show a very small contribution of orbital moment in the formation of total magnetic moments in both the compounds. The DOS of ferromagnetic ground state of ZrFe2 and Zr0.8Sc0.2Fe2 are interpreted on the basis of a covalent magnetic model beyond the Stoner rigid band model. It appears that on alloying between a magnetic and a non-magnetic partner (with low valence), a polarization develops on the non-magnetic atom which is anti-parallel to that of the magnetic atom.

Magnetic Compton profile evaluation of magnetization process of TbxCo100−x films

An understanding of the microscopic processes underlying the magnetization of the thin films is essential to the design and development of efficient magnetic recording media. In this context, we measure the magnetic Compton profile (MCP) of TbxCo100−x films (13 x 20) and investigate the microscopic (spin magnetic moment (), orbital magnetic moment (), and element specific magnetic moment ( and )) magnetization processes. Using the magnetic-field dependence of MCP, we obtain the spin- and orbital-specific magnetization curves (SSMH and OSMH, respectively). From the profile-fitting analysis, we also obtain the element specific SSMH curves, and we find from our results that the ratio is nearly constant. Thus, it is assumed that the magnetic switching processes of and cooperatively occur in this system (corresponding to and ). In addition, we observe that and are −1 at x 17. This implies that the magnetic moment of Tb aligns toward the magnetic field in the region of x > 17 and that of Co aligns toward the magnetic field in the region of x < 17. We estimate that this value of x 17 is the compositional point that corresponds to the compensation composition of Tb and Co.

Temperature dependent magnetic Compton profiles and first-principles strategies of quaternary half-Heusler alloy Co1−xCuxMnSb(0 ⩽ x ⩽ 0.8)

Temperature dependent experimental Compton profiles of quaternary alloys Co1−xCuxMnSb (x = 0.0, 0.2, 0.6 and 0.8) when decomposed into constituent profiles show that the sp-electron spin polarization is antiferromagnetically coupled to Mn-3d moments. The orbital magnetic moments derived from combination of magnetic Compton profiles (MCPs) and magnetization measurements are found to be small. Moreover, the first-principles full potential-linearized augmented plane wave (FP-LAPW) calculations have been performed to validate the experimental investigations of spin moments and half-Heusler properties. Present experimental and theoretical work show major role of Mn atoms in building-up the absolute spin moments. Our MCP data and spin-projected density of states derived from FP-LAPW computations show an increase in sp-d interaction in conduction region on increasing the Cu concentration. Further, Ruderman–Kittel–Kasuya–Yosida-type hybridization and antiferromagnetic superexchange interactions are witnessed in the reported alloys.

Spin density in YTiO3 : II. Momentum-space representation of electron spin density supported by position-space results

Unpaired electrons in YTiO 3 ferromagnetic crystal (below 30 K) were studied by polarized neutron diffraction (PND) and incoherent x-ray magnetic Compton scattering (MCS). These experiments provide both position and momentum representations of the electrons at the origin of the magnetic behavior, mostly those in the t 2g state of Ti atoms. A two-dimensional reconstruction was conducted from experimental and theoretical directional magnetic Compton profiles to obtain the two-dimensional magnetic electron momentum density. A superposition method is proposed to examine the coherence between results for position and momentum spaces, respectively. This model-free approach allows a straightforward cross-checking of PND and MCS experiments. An isolated Ti model is proposed to emphasize the role played by O 1 in the ferromagnetic coupling between Ti and its neighboring atoms.

Spin/orbital and magnetic quantum number selective magnetization measurements for CoFeB/MgO multilayer films

Spin selective magnetic hysteresis (SSMH) curves, orbital selective magnetic hysteresis (OSMH) curves and magnetic quantum number selective SSMH curves are obtained for CoFeB/MgO multilayer films by combining magnetic Compton profile measurements and superconducting quantum interference device (SQUID) magnetometer measurements. Although the SQUID magnetometer measurements do not show perpendicular magnetic anisotropy (PMA) in the CoFeB/MgO multilayer film, PMA behavior is observed in the OSMH and SSMH curves for the |m| = 2 magnetic quantum number states. These facts indicate that magnetization switching behavior is dominated by the orbital magnetization of the |m| = 2 magnetic quantum number states.